1. Field of the Invention
The present invention relates to a semiconductor fabrication process. More particularly, the present invention relates to a method of fabricating a dynamic random access memory (DRAM).
2. Description of the Related Art
Dynamic random access memory (DRAM) is a type of nonvolatile and easy-to-access memory mostly for holding operating data in a computer. Typically, a DRAM consists of an array of cells each comprising a metal-oxide-semiconductor (MOS) transistor and a capacitor. The source/drain regions of the transistor are electrically connected to a capacitor and a bit line respectively. At present, DRAM capacitors are classified into stacked capacitor or trench capacitor. A stacked capacitor is formed over the transistor and a trench capacitor is formed below the transistor.
To lower the sub-threshold current of the transistor and increase data retention capacity of storage electrode in a trench type DRAM capacitor, dosage level of the threshold voltage adjustment implantation and/or pocket implantation is often increased. The pocket implantation is a process of forming a doped pocket region on one side of a bit line connected source/drain region. However, this process also intensifies the rise in electric field at the PN junction and hence increases the leakage current there.